ipa-kdb: cache local TGS in the driver context
    
    For Kerberos principal lookup we always need to check whether principal
    is from our realm. Keep the reference to our realm TGS handy to avoid
    memory allocations on every lookup.
    
    Related: https://pagure.io/freeipa/issue/8319
    
    Signed-off-by: Alexander Bokovoy <abokovoy@redhat.com>
    Reviewed-By: Isaac Boukris <iboukris@redhat.com>
    Reviewed-By: Florence Blanc-Renaud <flo@redhat.com>